The production of semiconductor devices, such as semiconductor wafers, semiconductor substrates, flat panel displays, data discs and other similar articles, generally requires at least one step in which a coating must be applied in a uniform layer across a surface of the device. For instance, the production of integrated circuits frequently involves the application of a uniform coating of photoresist on a silicon wafer or substrate.
The small feature size and variety of micro-devices being produced need highly uniform coating layers to be produced. The production of micro-devices is significantly affected by current limitations associated with non-uniformity in coating layers, particularly when coating over irregular surfaces. Such irregular surfaces occur due to the micro-devices having one or more features such as vias, channels, and peaks. These features produce irregularities in the height of the surface over which the coating is being applied. These surface irregularities cause problems and limit the overall production performance and effectiveness of conventional coating apparatus and methods because the coatings cannot be applied in a sufficiently uniform manner. The coatings often fill channels, run off the peaks, and in some instances are unable to adequately fill vias. As a result, the coating layer is thickened in the channels, and thinned on the peaks. Vias can either be underfilled or overfilled depending upon viscosity and feature geometry.
A common prior art technique for applying photoresist coatings involves spraying the wafer with a photoresist and then spinning the wafer. The spinning action of the wafer produces centrifugal forces which spread the liquid photoresist. However, these spin application techniques have difficulties in providing layers having good uniformity. Striations are a common problem. These striations can be initiated by surface features, contaminants, or fluid properties of the coating being applied. These and other irregularities have derogatory effects on the production of micro-circuits and other micro-devices.
Prior art semiconductor coating techniques have not been able to provide thin, uniform coating layers which conform to irregularities present on the wafer or other semiconductor surface being coated. Spin coating techniques produce coating layers which tend to have an approximately level or planar surface even though surface features of varying heights are contained beneath the coating. The surfaces of wafers can contain topographical height variations of 10-40 microns with associated horizontal increments of 100-500 microns. Coatings thicknesses can thus vary in the range of 5-30 microns. This creates variations in the width of lines or other critical dimensions. These variations can in turn cause significant process yield losses. Thus, there is a need for improved coating apparatus and methods which can produce a coating layer onto semiconductor surfaces which is conformational to provide more uniform coating thickness, even when applied over surfaces having features of varying heights and shapes.
Prior art coating techniques have also been troubled by difficulties which arise during lithographic processes performed upon coating layers. These difficulties arise when coating thicknesses vary to a degree sufficient to cause focusing variations in the lithographic beams used to define features of a device. These problems are in particular significant when complex topographical configurations are used. This increased difficulty occurs due to the greater difficulty in producing uniform coating thicknesses on complex topographical configurations.
Prior art semiconductor coating equipment and techniques have also been deficient in not providing uniform application of relatively viscous coating materials. The exact mechanism causing the difficulties are not fully understood. This problem of coating with viscous coatings is further exacerbated when the surface being coated is irregular, such as discussed above.
The application of coatings to semiconductor article surfaces is further complicated by the extraordinarily low levels of contamination which must be maintained when processing semiconductor materials. Contaminating particles will cause defects to exist in the resulting products and will typically decrease device yields and profitability. Thus there is a strong need to produce uniform coating layers free from contaminants or congealed particulate accumulations which may form from the coating materials themselves.
Another problem associated with present equipment and methods for coating semiconductor wafers and similar devices is that a relatively large volume of coating material is used. This occurs in some instances because the coating is applied and the wafer is spun to provide centrifugal dispersion of the coating across the wafer surface. This leads to coating material being spun off and wasted. In other equipment the coating spray is not efficiently applied and is wasted in part as an aerosol of coating particles which do not adhere to the surface being coated.
A further problem associated with current techniques is inefficient coating application equipment and techniques. The excess coating material is either wasted, or else time and money are expended to dispose of, reformulate, or recycle the spent coating material. Thus there is a continuing need for methods and apparatus which can more precisely coat such articles using a relatively smaller amount of coating material and with reduced waste.
For these and other reasons, there is a strong need for improved methods and apparatus which can provide a uniform coating layer on irregular semiconductor article surfaces using reduced amounts of the coating materials.